Left panel: example of the dereddening algorithm, coded with the black dashed line, for a star with colors compatible with the reddened CTTSs locus provided by Meyer et al. (1997) (the green solid line). The blue solid line is the 2 Myr old isochrone suitable for the ONC. The red dashed lines are parallel to the reddening direction and bracket the regions occupied by the previous loci reddened by any A_V. Right panel: example of the reddening algorithm for a star with NIR colors compatible with reddened photospheric colors. The blue line represents the isochrone in the (J, J − H) diagram.